The present invention relates to the formation of filament or fiber reinforced titanium-based composite materials. More particularly it relates to the process of formation on a continuous basis of titanium base alloy matrix composites containing silicon carbide filaments or fibers or similar high temperature high strength filaments as a reinforcing material.
Such materials have been identified as potential materials of high specific strength, that is high strength to weight materials, making them attractive for use in future aircraft engines having a high trust to weight ratios.
It is anticipated that Ti.sub.3 Al and other titanium base alloy matrix composites will find application in wound rotors, casings and other intermediate temperature high stress applications. At present titanium base alloy matrix composites have been fabricated by rolling titanium base ingot to 0.010 inch thick sheet and then laying up alternate layers of titanium base alloy sheet with SiC fibers to form a laminate. The laminate is then consolidated by hot pressing or HIPing. The present process is believed to be inadequate for achieving high rates of production in industrial manufacturing. It is also believed to be too expensive for use in high rate production of such laminates.
As used herein the term titanium base alloy means an alloy composition in which titanium is at least half of the composition in parts by weight when the various alloy constituents are specified in parts by weight as, for example, in percentage by weight.
A titanium-aluminum intermetallic compound is a titanium base alloy composition in which titanium and aluminum are present in a simple numerical atomic ratio and the titanium and aluminum are distributed in the composition in a crystal form which corresponds approximately to the simple numerical ratio such as 3:1 for Ti.sub.3 Al, 1:1 for TiAl; and 1:3 for TiAl.sub.3.
Ti.sub.3 Al compositions have use temperatures of up to about 1400.degree. F. as compared to the use temperatures of titanium alloys such as Ti-6Al-4V of up to about 1000.degree. F. The use temperatures of TiAl is in the 1700.degree.-1800.degree. F. range.
A copending application Ser. No. 010,882 referenced above describes a method for overcoming many of the difficulties associated with a laminate sheet approach for fabricating titanium base alloy composites. The text of the copending application is incorporated herein by reference. The method described in copending application Ser. No. 010,882 employs a cylindrical drum and provides a closely spaced winding of silicon carbide fibers on the drum. The wound drum is then coated with a layer of titanium base alloy employing a rapid solidification low pressure plasma deposition process and particularly a process employing radio frequency energy for the plasma deposition process. The process forms a metal impregnated silicon carbide filament tape which serves as one of several layers for fabrication of a metal matrix composite layup. The layup is then consolidated by hot pressing or HIPing.
The method of copending application Ser. No. 010,882 offers a cost and performance benefit relative to the prior art preformed laminate sheet approach. However, the method of plasma forming the integral individual sheets of the copending application is not a continuous method of fabricating fiber containing metal impregnated sheet and for this reason is inferior to the subject process.
Novel and unique structures are formed by novel methods pursuant to the present invention by plasma spray deposit of titanium base alloys and titanium-aluminum intermetallic compounds employing RF plasma spray apparatus on a continuous basis.
The formation of plasma spray deposits of titanium and of alloys and intermetallic compounds of titanium present a set of processing problems which are unlike those of most other high temperature high strength materials such as the superalloys. A superalloy such as a nickel base, cobalt base, or iron base superalloy can be subdivided to relatively small size particles of -400 mesh (about 37 .mu.m) or smaller without causing the powder to accumulate a significant surface deposit of oxygen. A nickel base superalloy in powder form having particle size of less than -400 mesh will typically have from about 200 to about 400 parts per million of oxygen. A powdered titanium alloy having the same particle size will by contrast typically have a ten fold higher concentration of oxygen. A powdered titanium alloy of -400 mesh will have between about 2000 and 4000 ppm of oxygen.
Moreover titanium alloy powder of less than -400 mesh size is recognized as being potentially pyrophoric and as requiring special handling to avoid pyrophoric behavior.
It is also recognized that the ductility of titanium alloys decreases as the concentration of oxygen and of nitrogen which they contain increases. It is accordingly important to keep the oxygen and nitrogen content of titanium base alloys at a minimum.
Prior art plasma spray technology is based primarily on use of direct current plasma guns. It has been recognized that most plasma spray deposits of the superalloys such as nickel and iron base superalloys have had relatively low ductility and that such deposits when in their as-deposited form can be cracked when bent through a sufficiently acute angle due to the low ductility.
I have discovered that RF plasma apparatus is capable of spraying powder of much larger particle size than the conventional d.c. plasma apparatus. I have discovered that particle sizes at least three times larger in diameter than those conventionally employed in d.c. plasma spray apparatus may be successfully employed in plasma spray practices and that the particle size may be as high as 100 .mu.m to 250 .mu.m and larger and as large as 10X as large as the -400 mesh powder previously employed in d.c. plasma spray practice.
This possibility of employing the larger powder particles is quite important for metal powders such as titanium which are subject to reaction and absorption of gases such as nitrogen and oxygen on their surfaces. One reason is that the surface area of particles relative to their mass decreases inversely as their diameters. Accordingly a three fold increase in particle diameter translates into a three fold decrease in particle surface area. I have discovered that one result is that RF plasma spray deposited structures of titanium base alloys made with the aid of larger particles have lower oxygen content than might be expected based on knowledge of prior art practices.